The present invention relates to an inspection unit for a high frequency/high-speed device for ensuring reliable connection between the inspection unit and the device to be inspected, on occasion of inspecting its electrical performance, before a module of a high frequency/high-speed circuit such as an amplifier circuit, a mixer circuit, a filter circuit, a memory, a CPU, etc. or an IC to be incorporated in a mobile phone, for example, has been assembled to a circuit board. In this specification, the term “high frequency” refers to an analogue signal having a high frequency (1 GHz or more), while the term “high-speed” refers to a digital signal having very short pulse width and pulse interval, and both of which are hereinafter collectively referred to as RF (radio frequency).
On occasion of inspecting electrical performance of the RF device such as a semi-conductor wafer, an IC, or a module, insufficient contacts between the terminals may particularly cause fluctuation of impedance or other measurement factors, which may sometimes vary to change measured values. Under the circumstances, such inspection has been conducted by a special inspection unit, for example, as shown in FIG. 4A (disclosed in Japanese Patent Publication No. 2001-99889A). In such an inspection, an RF circuit, which is the device to be inspected, is constructed in a form of a module 50 including an amplifier circuit and a mixer circuit, and is housed in a metal casing for avoiding interference with the exterior. The module 50 includes input and output terminals 51, 54 for RF signals, a power supply electrode terminal 52, and a grounding terminal 53, which are provided on a back face of the metal casing. Then, the inspection is conducted by electrically connecting the terminals to respective terminals of a wiring board 66 on which certain wirings for the inspection are arranged.
In the example as shown in FIG. 9A, there are employed contact probes each having a spring and a plunger contained in a metal pipe, one end of the plunger being adapted to be projected to the exterior by the spring and contracted when pushed. The respective electrode terminals are connected by contact probes 63 for RF signals, a contact probe 64 for power supply, and a contact probe 65 for grounding which are contained in a metal block 61 for preventing them from being affected by noises. Each of the contact probes 63 for RF signals is formed in a coaxial structure, using the contact probe as a core conductor and using an inner wall of a through hole in the metal block 61 as an outer conductor, especially for preventing intrusion of noises. In FIG. 9A, denoted with numeral 67 is a coaxial cable, and 68 is a plate for pressing the metal pipes surrounding the contact probes. Such a structure around the contact probes is almost the same in case of an IC socket for inspecting ICs, though such socket has a different outer shape.
Although FIG. 9A shows only two contact probes 63 for RF signals (for input and output), and one each contact probes 64, 65 each for power supply and for grounding, a large number of these contact probes are actually provided in the metal block 61. In the maximum case, the electrode terminals of about 600 pieces per 1 cm2 are provided in a matrix manner with a narrow pitch of about 0.4 mm.
In such the narrow-pitch device, an outer diameter of the contact probe for RF signals including a dielectric layer must be reduced in size. Meanwhile, it is also necessary to adjust the impedance of the coaxial structure formed by the contact probe and the inner wall of the through hole to a predetermined characteristic impedance (50 Ω, for example) satisfying the following Equation (1).
                    Zo        =                              60                                          ɛ                r                                              ⁢                      log            e                    ⁢                      D            d                                              (        1        )            where, d is the outer diameter of the core conductor (the contact probe), D is the inner diameter of the outer conductor (the through hole), and εr is a dielectric constant of the dielectric substance between them.
In order to satisfy the Equation (1), it is possible to reduce the inner diameter D of the outer conductor by providing a tube made from dielectric substance with small dielectric constant between each contact probe and each through hole. However, even though a tube of polytetrafuluoroethylene having dielectric constant of 2.1, which is the dielectric substance having the smallest dielectric constant available at present, is employed, and the contact probe having the smallest diameter available (having the outer diameter of 0.15 mm) is employed, the inner diameter of the outer conductor (the inner diameter of the through hole formed in the metal block) requires about 0.5 mm to obtain 50Ω as the characteristic impedance of the coaxial structure. This cannot attain the pitch of 0.4 mm.
For the purpose of solving the problem as described above, Japanese Patent Publication No. 2004-170182A discloses such a structure that dielectric rings 69 are fixed to at least two positions of an outer periphery of each contact probe 63 as shown in FIG. 9B, thereby to concentrically hold the contact probe in a through hole of a metal block with a gap. According to this structure, almost all parts of the dielectric substance between the contact probe as the core conductor and the inner face of the through hole of the metal block as the outer conductor can be formed to be an air layer having dielectric constant of 1. That is, it is possible to decrease the dielectric constant between the core conductor and the outer conductor of the coaxial structure, thereby to reduce a hole diameter of the through hole as compared with the contact probe having the same size, and hence, narrowing the pitch can be achieved.
However, in order to firmly hold the contact probe in the through hole by the dielectric rings, it is necessary to make outer diameters of the dielectric rings somewhat larger than the inner diameter of the through hole, and to forcibly push the contact probe with the dielectric rings into the through hole. The contact probe itself is in a shape of a very small pipe having an outer diameter of about 0.15 mm, and it is difficult to push it into the through hole. In this respect, there has been a problem that the contact probe is liable to be crooked with an external force, and even with a little crook, smooth movement of the plunger to be achieved by the spring within the pipe may be hindered. Moreover, in a case where the dielectric ring is made larger in width with a view to enhancing easiness in its insertion process and mechanical strength, the dielectric constant of the dielectric substance in the coaxial structure becomes larger, resulting in disturbance of the characteristic impedance, which will sacrifice the high frequency characteristics and high speed performance.
Meanwhile, Japanese Patent Publication No. 2004-325306A discloses a structure for holding such small contact probes in the through holes in the metal block as shown in FIG. 8A. In this structure, a metal block 2 and a metal plate 31 are formed with recessed parts, and insulative spacers 32 are inserted into the recessed parts concentrically with the through holes so that opposite ends of the contact probes 1 can be held by the insulative spacers 32. According to this structure, the contact probes 1 will hardly receive external forces, and can be held concentrically with the through holes 21 keeping gaps from the through holes, thereby to decrease the relative dielectric constant and to attain pitch narrowing.
However, this structure has also a problem that when the metal plate 31 is mounted after the contact probes 1 have been inserted, the contact probes 1 in the coaxial structure may be inclined, as shown in FIG. 8B, and a work for mounting the metal plate 31 is difficult because several hundreds of the contact probes including the contact probes for power supply and grounding are present within an area of a few cm2, and each the contact probe in the coaxial structure has a diameter of about 0.15 mm and a length of a few millimeter.